1. Field of the Invention.
The invention concerns a reversible three-phase rotary electrical machine of the alternator starter type.
2. Description of Related Art.
In this type of machine functioning as a starter, various coils evenly spaced apart on the periphery of part of the machine (in general the stator) are supplied successively by a set of switches in order to generate a rotating magnetic field that causes the driving of the other part of the machine (in general the rotor), for its part supplied continuously.
For example, in the case of a delta configuration of the coils (or phases) of the stator, the coils are supplied by the set of switches so as to subject each of the nodes common to two coils, either to a first voltage (for example positive) or to a second voltage (for example zero). Suitable control of the switches thus makes it possible to reverse the current passing through each coil during a rotation period of the machine and therefore to generate the rotating field mentioned above.
In order to generate the control signals for the switches aimed at obtaining such a control, one practical solution consists of placing equally distributed sensors around the rotary machine (here three sensors since it is a case of a three-phase machine) and controlling the switches by means of signals generated by these sensors (directly for the switches applying the first voltage, and with an inversion of the signal for the switches applying the second voltage), according to a technique in general referred to as “full-wave control”.
However, according to this solution, the signals issuing from the sensors consisting of two half waves (one positive, the other negative) each extending over half of the period of the signal (that is to say over 180° in terms of phase), the following configuration is obtained at each moment: each node situated between two coils in a delta configuration is connected to one of the voltage sources (because at each moment one of the switches associated with it is closed) and two nodes out of the three are thus connected to the same voltage source and thus short-circuit the coil that separates them.
Because of this, the equivalent resistance of the rotary machine is only equal to half of the resistance of a coil, which may pose a problem under certain operating conditions.
This is for example the case with rotary machines intended to assist a thermal engine during its acceleration phases and the coils of which are sized with a low stator resistance in order to supply high mechanical power at high speed. The currents involved in the low resistance may prove to be destructive for the electronics in other operating circumstances, such as for example starting the thermal engine at high voltage.
The problems disclosed above have been presented in the particular case of a stator having a delta coil configuration. It should be noted however that these problems also exist in the case of a star configuration.